of the complex expression semantically depending only on this, not on that. A
demon evaluator who custom-picked the evaluation order to make things maximally
bad for you could ensure that all the semantically unnecessary computations got
-evaluated anyway. We don't have any way to prevent that. Later,
-we'll see ways to *semantically guarantee* one evaluation order rather than
-another. Though even then the demonic evaluation-order-chooser could make it
-take unnecessarily long to compute the semantically guaranteed result. Of
-course, in any real computing environment you'll know you're dealing with a
-fixed evaluation order and you'll be able to program efficiently around that.
+evaluated anyway. We don't yet know any way to prevent that. Later, we'll see
+ways to *guarantee* one evaluation order rather than another. Of
+course, in any real computing environment you'll know in advance that you're
+dealing with a fixed evaluation order and you'll be able to program efficiently
+around that.
In detail, then, here's what our v5 lists will look like:
> that `5` is the outer/leftmost head of the list. That's not an efficient way
> to get the leftmost head.
>
-> We could improve this by building lists as left folds when implementing them
-> as continuation-passing style folds. We'd just replace above:
+> We could improve this by building lists as **left folds**. What's that?
+>
+> Well, the right fold of `f` over a list `[a;b;c;d;e]`, using starting value z, is:
+>
+> f a (f b (f c (f d (f e z))))
+>
+> The left fold on the other hand starts combining `z` with elements from the left. `f z a` is then combined with `b`, and so on:
+>
+> f (f (f (f (f z a) b) c) d) e
+>
+> or, if we preferred the arguments to each `f` flipped:
+>
+> f e (f d (f c (f b (f a z))))
+>
+> Recall we implemented v3 lists as their own right-fold functions. We could
+> instead implement lists as their own left-fold functions. To do that with our
+> v5 lists, we'd replace above:
>
> let make_list = \h t. \f2 z continue_handler abort_handler.
> f2 h z (\z. t f2 z continue_handler abort_handler) abort_handler
>
-> now `extract_head` should return the leftmost head directly, using its
-> `abort_handler`:
+> Having done that, now `extract_head` can return the leftmost head
+> directly, using its `abort_handler`:
>
> let extract_head = \lst larger_computation. lst
> (\hd sofar continue_handler abort_handler. abort_handler hd)